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The Domain Name System (DNS) has a tree structure or hierarchy, with each non-RR (resource record) node on the tree being a domain name. A subdomain is a domain that is part of a larger domain; the only domain that is not also a subdomain is the root domain.[1] For example, west.example.com and east.example.com are subdomains of the example.com domain, which in turn is a subdomain of the comtop-level domain (TLD). A "subdomain" expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org comprises a subdomain of the domain wikipedia.org. In theory this subdivision can go down to 127 levels deep, and each DNS label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 255 characters. But in practice most domain registries limit at 253 characters.[citation needed]

Subdomains in this context are defined by editing the DNS zone file pertaining to the parent domain. However, there[where?] is a lively debate over the use of the term “subdomain” when referring to names which map to the Address record (A; host) and various other types of zone records which may map to any public IP address destination and any type of server. Certain[which?] groups insist that it is inappropriate to use the term “subdomain” to refer to any mapping other than that provided by zone NS (name server) records and any server-destination other than that of a domain name server. Notwithstanding the terminology debate, many prominent public DNS providers[which?] use the term “subdomain” to refer to names which map to A (host) records which may map to any type of host or destination-server.[citation needed]

Subdomains are also used by organizations that wish to assign a unique name to a particular department, function, or service related to the organization. For example, a university might assign "cs" to the computer science department, such that a number of hosts could be used inside that subdomain, such as mail.cs.example.edu or www.cs.example.edu.

Depending on application, a record inside a domain, or subdomain might refer to a hostname, or a service provided by a number of machines in a cluster. Some websites use different subdomains to point to different server clusters. For example, www.example.com points to Server Cluster 1 or Datacentre 1, and www2.example.com points to Server Cluster 2 or Datacentre 2 etc..

Some domains host their nameservers as ns1.example.com, ns2.example.com, etc., and these usually never show up in search engine results.